The invention relates to a process for preparation of hydrophilic or partially hydrophilic inorganic aerogels from hydrophobic inorganic aerogels.
1. Field of the Invention
Aerogels, in particular those with porosities of greater than 60% and densities of less than 0.6 g/cm3, have extremely low thermal conductivity and are therefore used as thermal insulation material, for example as described in EP-A-0 171 722.
2. Description of the Related Art
Aerogels in the wider sense, ie. in the sense of xe2x80x9cgel with air as dispersion mediumxe2x80x9d are prepared by drying a suitable gel. The term xe2x80x9caerogelxe2x80x9d in this sense is taken to mean aerogels in the narrower sense, xerogels and cryogels, a dried gel being termed an aerogel in the narrower sense if the gel liquid is removed to a very large extent at temperatures above the critical temperature and starting at pressures above the critical pressure. If, in contrast, the gel liquid is removed subcritically, for example with formation of a liquid-vapor boundary phase, then the resultant gel is termed a xerogel.
In the present application, the term aerogel is taken to mean aerogels in the wider sense, ie. in the sense of xe2x80x9cgel with air as dispersion mediumxe2x80x9d.
This term is taken to exclude xerogels known from the earlier literature, for example those obtained by precipitation of silica (eg. DE-A-30 25 437 and DD-A296 898), or produced in the form of pyrogenic silica (eg. Aerosil(copyright)). In these cases, however, the preparation does not give a homogeneous three-dimensional gel structure which extends across relatively large distances.
Aerogels may fundamentally be divided into inorganic and organic aerogels.
Inorganic aerogels were known as early as 1931 (S. S. Kistler, Nature 1931, 127, 741). Since then, aerogels have been prepared from a wide variety of starting materials. For example, SiO2 aerogels, Al2O3 aerogels, TiO2 aerogels, ZrO2 aerogels, SnO2 aerogels, Li2O aerogels, CeO2 aerogels, V2O5 aerogels and mixtures of these have been prepared (H. D. Gesser, P. C. Goswami, Chem. Rev. 1989, 89, 765ff). Organic aerogels made from a wide variety of starting materials, for example from melamine formaldehyde, have also been known for some years (R. W. Pekala, J. Mater. Sci. 1989, 24, 3221).
Inorganic aerogels can be prepared by two fundamentally different methods.
Firstly, SiO2 aerogels, for example, can be prepared by acid hydrolysis and condensation of tetraethyl orthosilicate in ethanol. This gives a gel which can be dried supercritically with retention of the structure. Preparation processes based on this drying technique are known, for example, from EP-A-0 396 076, WO 92/03378 and WO 95/06617.
A fundamental alternative to supercritical drying is provided by a process for subcritical drying of SiO2 gels, in which the SiO2 gel can be obtained, f or example, by acid hydrolysis of tetraalkoxysilanes in a suitable organic solvent, using water. After exchanging the solvent for a suitable organic solvent, the resultant gel is reacted with a silylating agent, in a further step. The SiO2 gel thus obtained can then be dried from an organic solvent in air. Aerogels with densities of less than 0.4 g/cm3 and porosities of greater than 60% can be attained in this way. The preparation process based on this drying technique is described in detail in WO 94/25149.
The gels described above may moreover be mixed with tetraalkoxysilanes and aged before drying in the aqueous alcoholic solution, in order to increase the strength of the gel structure, for example as disclosed in WO 92/20623.
However, the tetraalkoxysilanes used as starting materials in the processes described above are exceptionally costly. A considerable reduction in costs can be achieved by using water glass as starting material for preparing the SiO2 gels, for example starting with an aqueous water glass solution and using an ion-exchange resin to prepare a silica which polycondenses on addition of a base to give an SiO2 gel. After exchanging the aqueous medium for a suitable organic solvent, the resultant gel is then reacted with a chlorine-containing silylating agent, in a further step. The SiO2 gel thus obtained, modified on its surface with, for example, methylsilyl groups, can then likewise be dried in air from an organic solvent. The preparation process based on this technique is known from DE-A-43 42 548. Alternative processes relating to the preparation of an SiO2 hydrogel based on water glass followed by subcritical drying are described in the German Patent Applications 195 41 715.1 and 195 41 992.8.
German Patent Application 195 02 453.2 moreover describes the use of chlorine-free silylating agents in the preparation of subcritically dried aerogels.
German Patent Application 195 34 198.8, furthermore, describes an organofunctionalization using organo-functionalized silylating agents in the preparation of subcritically dried aerogels.
EP-A-0 606 638 discloses, furthermore, the preparation of SiO2 aerogels containing carbon particles by heating organically modified SiO2 aerogels in the presence of at least one pyrolyzable hydrocarbon gas and/or at least one inert gas. The organic groups on the surface of the aerogels and/or those of the hydrocarbon gas are oxidized here in an oxygen-free atmosphere to give elemental carbon.
Depending on the solvent used in the supercritical drying, the aerogels obtained by supercritical drying are hydrophilic because of OH groups on the internal surface (supercritical drying from CO2) or hydrophobic in the short term because of alkoxy groups on the internal surface (supercritical drying from alcohols). However, in the long term a reaction of the alkoxy groups with water from the surroundings takes place and leads to formation of hydroxide groups on the internal surface, again giving a hydrophilic aerogel.
The formation of a hydrophilic aerogel can be avoided by a hydrophobicization step during the supercritical drying. For this, a hydrophobicizing agent, for example Me2Si(OMe)2, can be used, as disclosed in EP-A-0 396 076, for example. The methylsilyl groups on the internal surface make the resultant aerogels permanently hydrophobic.
As a result of the preparation process (silylation before drying), subcritically dried aerogels are permanently hydrophobic. Since the subcritical drying of aerogels requires unreactive, hydrophobic internal gel surfaces, direct preparation of hydrophilic, subcritically dried aerogels is impossible.
For many applications, however, a hydrophilic or partially hydrophilic internal surface of the aerogels is precisely what is essential or advantageous.
It was therefore an object of the present invention to provide a process for preparation of hydrophilic or partially hydrophilic inorganic aerogels, but without having to accept the disadvantage of supercritical drying, with its disproportionately high technical complexity.
This object is achieved by means of a process in which a hydrophobic inorganic, preferably subcritically dried, aerogel is pyrolyzed in the presence of oxygen at temperatures in the range from 100 to 1000xc2x0 C.
Not Applicable
For the purposes of the present application, the term xe2x80x9cthe presence of oxygenxe2x80x9d is taken to mean that a sufficient amount of oxygen is present to oxidize the surface groups of the aerogels, it being possible for this amount to be present in any formulation known to the person skilled in the art (e.g.: air, oxygen-air mixture and/or oxygen-inert gas mixture).
For the purposes of the present application, the term xe2x80x9cinorganic aerogelxe2x80x9d is taken to mean an aerogel whose preparation has been based on inorganic materials.
The term xe2x80x9caerogels based on inorganic materialsxe2x80x9d is taken, in particular, to include those aerogels which have been modified, for example, by silylation.
Preference is given to aerogels having hydrophobic surface groups, said aerogels consisting predominantly of SiO2, Al2O3, TiO2, ZrO2, or mixtures of these. Such aerogels having hydrophobic surface groups can be prepared by any process known to the person skilled in the art. Particular preference is given to hydrophobic SiO2-containing aerogels, in particular SiO2 aerogels.
In a preferred embodiment, the aerogel is pyrolyzed at temperatures from 100 to 1000xc2x0 C., particularly preferably at from 150 to 800xc2x0 C., in particular at from 250 to 650xc2x0 C., in a gentle flow of air.
The duration of the pyrolysis is essentially determined by the surface modification and the density of the material of the aerogels. The duration of the pyrolysis is preferably less than 10 hours, particularly preferably less than 1 hour.
If partially hydrophilic aerogels are to be prepared, the temperature and the duration of the pyrolysis should be adjusted so that only an appropriate proportion of the organic surface groups pyrolyzes.
If the aerogels have different organic groups on their internal surface, the different decomposition temperature of the individual organic groups can be used to pyrolyze the appropriate groups in a controlled fashion and in accordance with the desired degree of hydrophilicity. In the case of aerogels having both phenyl and methyl groups on their internal surface, for example, properties which are partly hydrophilic can then be achieved by pyrolysis at 450xc2x0 C., which substitutes hydroxide groups for the methyl groups on the surface of a corresponding aerogel, but not for the phenyl groups which are present.